In a Multiple-Input Multiple-Output (MIMO) system, spatial multiplexing allows the transmission of multiple data streams (or data layers) over different spatial channels. Multiple transmit antennas can send different data streams over separate spatial channels, and the use of multiple receive antennas can allow the recovery of the different data streams. FIG. 1 shows a 2×2 MIMO communication system in which two transmit antennas (Tx Ant1 and Tx Ant2) transmit signals to two receive antennas (Rx Ant1 and Rx Ant2). As shown schematically in FIG. 1, each transmit antenna (Tx Ant) transmits to all receive antennas (Rx Ant) at the receiver. In other MIMO communication systems any number of transmit antennas and receive antennas may be used, and the maximum number of data streams that can be distinguished due to the spatial multiplexing is equal to the lower of the number of transmit antennas and the number of receive antennas.
The transmission system shown in FIG. 1 can be described by the equationr=Hx+n  (1)where r denotes the received signal vector, x is the transmitted signal vector, H indicates the MIMO channel matrix, and n is the noise (noise-plus-interference) vector. The channel matrix H models the characteristics of the MIMO propagation channel. In the case of a frequency non-selective channel, Equation (1) can be expanded for a 2×2 MIMO channel as
                                          [                                                                                r                    1                                                                                                                    r                    2                                                                        ]                    =                                                    [                                                                                                    h                        11                                                                                                            h                        12                                                                                                                                                h                        21                                                                                                            h                        22                                                                                            ]                            ⁡                              [                                                                                                    x                        1                                                                                                                                                x                        2                                                                                            ]                                      +                          [                                                                                          n                      1                                                                                                                                  n                      2                                                                                  ]                                      ,                            (        2        )            where r1 and r2 are the signals received at the respective receive antennas Rx Ant1 and RX Ant2; x1 and x2 are the signals transmitted from the respective transmit antennas Tx Ant1 and Tx Ant2; h11, h12, h21 and h22 are the coefficients of the (frequency non-selective) MIMO channel; and n1 and n2 are the noise (noise-plus-interference) at the respective receive antennas Rx Ant1 and Rx Ant2.
In one system, the transmitter uses a precoding matrix W to process N data streams for transmission to the receiver in the MIMO system. The transmitter signals to the receiver the values of W and N that have been used, thereby enabling the receiver to correctly decode the data streams. For example, the receiver may implement an equalizer as part of the decoding process for decoding the received data streams. An effective channel G at the output of the equalizer is calculated using the values of W and N indicated by the transmitter.
The receiver estimates its currently preferred precoding matrix Wp, its preferred number of data streams Np to be used in the MIMO system and a channel quality indicator (CQI) for each of the Np data streams corresponding to Np distinct transport blocks. The receiver reports these parameters (Wp, Np and the CQI(s)) to the transmitter. Although the values of Wp and Np are reported to the transmitter, the transmitter may use different values for the precoding matrix W and the number of transmitted data streams N when processing the data streams for transmission to the receiver. In other words, W may be different to Wp, and N may be different to Np.
In the system described above, the CQI(s) are calculated at the receiver based on an effective channel Gp that would be seen at the output of the equalizer if the transmitter used the preferred precoding matrix Wp and the preferred number of data streams Np (that is, if W=Wp and N=Np).